Isaacs equation
The Isaacs equation is the equality \[ \sup_{a \in \mathcal{A}} \ \inf_{b \in \mathcal{B}} \ L_{ab} u(x) = f(x), \] where $L_{ab}$ is some family of linear integro-differential operators with two indices $a \in \mathcal A$ and $b \in \mathcal B$.
The equation appears naturally in zero sum stochastic games with Levy processes.
The equation is uniformly elliptic with respect to any class $\mathcal{L}$ that contains all the operators $L_{ab}$. Under some conditions on that class, there are interior $C^{1,\alpha}$ estimates for the solution.
Note that any second order fully nonlinear uniformly elliptic PDE $F(D^2 u)=0$ can be written as an Isaacs equation by the following two steps:
- $F(X)$ is Lipschitz with constant $\Lambda$, so it is the infimum of all cones $C_{X_0}(x) = F(X_0) + \Lambda|X-X_0|$.
- Each cone $C(X)$ is the supremum of all linear functions of the form $L(X) = F(X_0) + \mathrm{tr} \, A \cdot (X-X_0)$ for $||A||\leq \Lambda$.
A more general second order fully nonlinear uniformly elliptic PDE $F(D^2 u, Du, u, x)=0$ can also be written as an Isaacs equation if it is Lipschitz with respect to all parameters.
Under very general assumptions, fully nonlinear integro-differential equations can be written in the form of an Isaacs equation.
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